1. Field of the Invention
The present invention relates to an electromagnetic induction device such as, for example, a transformer utilizing an inverter and, more particularly, to the electromagnetic induction device of a type finding a principal application in, for example, driving a magnetron.
2. Description of the Prior Art
FIG. 27 illustrates an inverter-equipped high frequency heating apparatus such as, for example, an electronic oven, of a type disclosed in the Japanese Examined Patent Publication No. 7-40465. This known high frequency heating apparatus includes a rectifying circuit 62 for rectifying and smoothing an electric power from a commercial power source 61, an inverter 63 for converting the rectified and smoothed electric power into a high frequency alternating current of a frequency equal to or higher than 20 kHz, and a transformer 64 including a gapped core and having a primary winding 64p to which the high frequency alternating current is supplied from the inverter 63. The transformer 64 also has a secondary winding 64s, and a high frequency output voltage emerging from the secondary winding 64s of the transformer 64 is, after having been rectified and smoothed by a half-wave rectifying circuit 65, supplied as a direct current high voltage to a magnetron 66. The transformer 64 furthermore has a heater winding 64h for driving the magnetron 66 which, when receiving the direct current high voltage, generates microwaves.
The transformer 64 discussed above is shown in a sectional representation in FIG. 29. The known transformer 64 comprises a bobbin 70 on which the primary winding 64p, the secondary winding 64s and the heater winding 64h are wound therearound in an axially spaced relation to each other. This known transformer 64 also comprises generally U-shaped magnetic core pieces 71 and 72 each having a pair of legs and a bridge arm 71a or 71b connecting the legs together, and one of the legs of each magnetic core piece 71 and 72 is received within a cylindrical hollow 70s of the bobbin 70. The respective legs of the magnetic core pieces 71 and 72 received within the cylindrical hollow 70s are spaced from each other by a spacer 70g of a thickness G that is formed within the cylindrical hollow 70s to define a magnetic gap 73 between end faces of the pairs of the legs of the magnetic core pieces 71 and 72. In a condition so assembled, the magnetic core pieces 71 and 72 form a core assembly 75 of a generally rectangular shape having a generally rectangular center void, wherein a coupling coefficient between the primary and secondary windings 64p and 64s is within the range of 0.6 to 0.8 so that the secondary winding can have a leakage inductance. This structure of the known transformer makes no use of a high frequency choke coil on the side of the secondary winding that has hitherto been required in the inverter circuit for use with the magnetron.
It has, however, been found that the known transformer 64 discussed above has a problem. Specifically, since a magnetic circuit C is formed only on one side of the primary and secondary windings 64p and 64s (i.e., on a left side as viewed in FIG. 29) and since the respective bridge arms 71a and 71b of the core pieces 71 and 72 forming the magnetic circuit C extend parallel to each other while spaced a substantial distance from each other, a magnetic loss is significant and no strong magnetic flux can be obtained. For this reason, in order to secure a required output voltage, the number of turns of the primary and secondary windings 64p and 64s cannot be reduced. Accordingly, with the known transformer 64, if the width (as measured in a direction conforming to the longitudinal sense of the bobbin 70) of each of the primary and secondary windings 64p and 64s is reduced so that the resultant transformer can have a substantially flat configuration, the coil outer diameter (as measured in a direction perpendicular to the longitudinal sense of the bobbin 70) of each of the primary and secondary windings 64p and 64s tends to increase for the number of turns thereof necessitated to secure the required output voltage. The consequence is that the known transformer 64 is relatively bulky, having a relatively large transverse dimension as measured in a lateral direction conforming to the coil outer diameter. As such, the transformer 64 of the structure discussed above is incapable of being assembled compact and requires a relatively large space for mounting on a circuit substrate.
The above discussed transformer 64 has another problem. As discussed above, the transformer 64 has the spacer 70g for defining the gap 73, that is positioned at a location surrounded by the primary winding 64p, and also makes use of the generally U-shaped core pieces 71 and 72 wherein the legs of the core piece 71 have a different from that of the core piece 72 and wherein one of the legs of the core piece 71 and one of the legs of the core piece 72 are inserted into the cylindrical hollow 70s of the bobbin 70. Accordingly, the known transformer 64 requires two types of core pieces of different sizes and this leads to increase of the type of core pieces and, hence, that of the manufacturing cost. The high frequency heating apparatus constructed utilizing the transformer 64 of the structure shown in and described with particular reference to FIG. 29 is generally mounted on a circuit substrate of a relatively large size on which electric component parts connected to the transformer 64 such as a primary circuit including the rectifying circuit 62 and the inverter 63 and a secondary circuit including the half-wave rectifying circuit 65 as shown in FIG. 27 are formed. Considering that the transformer 64 has a relatively large transverse dimension as discussed hereinbefore, mounting of such transformer 64 requires a further increase of the size of the circuit substrate. Also, since the secondary circuit defines a high voltage generating circuit, the circuit substrate must have a correspondingly increased size so that the secondary circuit can be spaced a sufficient distance from the primary circuit and a ground to provide a sufficient electrical insulation therebetween. For these reasons, a circuit unit including the transformer 64 mounted on the circuit substrate requires a relatively large space for installation and, therefore, application thereof is limited, thereby constituting a cause of the high frequency heating apparatus incapable of being manufactured compact.
Accordingly, the present invention has been devised to substantially eliminate the above discussed problems and is intended to provide an electromagnetic induction device that can be assembled having a substantially flat configuration without incurring an increase of the transverse dimension.
In order to accomplish the foregoing object of the present invention, there is provided an electromagnetic induction device including a core assembly for defining a magnetic circuit and comprised of generally T-shaped or L-shaped first and second core pieces, a generally flat bobbin having an axial width and a radial size, the axial width being smaller than the radial size and also having a bore defined therein so as to extend in an axial direction of the bobbin, and a winding member mounted on the bobbin. The core legs of the first and second core pieces are inserted into the bore of the flat bobbin while the core arms of the first and second core pieces extend parallel to each other.
The term xe2x80x9cT-shapedxe2x80x9d referred to hereinbefore and hereinafter in connection with each of the core pieces is intended to mean the shape in a stereoscopic vision similar to the shape of a figure xe2x80x9cTxe2x80x9d and does not include the T-shape as viewed in a side representation of a disc having a leg secured at one end to a center of the disc so as to extend perpendicular to the disc. Similarly, the term xe2x80x9cL-shapedxe2x80x9d referred to hereinbefore and hereinafter in connection with each of the core pieces is intended to mean the shape in a stereoscopic vision similar to the shape of a figure xe2x80x9cLxe2x80x9d and does not include the L-shape as viewed in a side representation of a disc having a leg secured to an off-center peripheral portion of the disc so as to extend perpendicular to the disc.
According to the present invention, since no core piece is positioned laterally of the winding member and, therefore, the electromagnetic induction device can have a reduced lateral dimension as measured in a direction perpendicular to the axial direction of the winding member. Moreover, since the bobbin is of a flat configuration having a reduced axial width, the spacing between the core arms of the T-shaped core pieces can be reduced in size, making it possible to form a strong magnetic field whereby an excellent magnetic characteristic can be obtained. Also, since the core pieces have the same shape and size, the number of types of core pieces required to form the core assembly can advantageously be reduced, thereby reducing the manufacturing cost.
In a preferred embodiment of the present invention, the winding member may include primary and secondary windings mounted on the bobbin in axially spaced relation to each other and, at the same time, respective free ends of the core legs of the first and second core pieces may confront with each other to define a gap therebetween. According to this design, the presence of the gap is effective to provide the electromagnetic induction device having a characteristic in which a magnetic saturation takes place hardly.
In a preferred embodiment of the present invention, a coupling coefficient between the primary and secondary windings is set to a value within the range of 0.6 to 0.8. Selection of the coupling coefficient within the particular range is effective to eliminate the need to use a high frequency choke in a secondary circuit where the electromagnetic induction device of the present invention is utilized in a high frequency heating apparatus of an inverter type.
Also, in one preferred embodiment of the present invention, the winding member includes primary and secondary windings mounted on the bobbin in axially spaced relation to each other. The primary winding may have lead lines extending from respective opposite ends thereof and fitted with a terminal member adapted to be connected with a terminal piece, mounted on a circuit substrate, by screwing or insertion, whereas the secondary winding may have opposite ends fitted with respective pin terminals fixedly secured to the bobbin and adapted to be inserted into the circuit substrate. This design is effective to allow the primary winding, generally prepared from a thick electric wire, to be easily connected to the circuit substrate. Also, since the opposite ends of the secondary winding prepared generally from a thin electric wire are connected with the pin terminals fixedly mounted on the bobbin, there is no possibility that one or both of the opposite ends of the secondary winding from which a high voltage is generated may accidentally fly during connection of the electromagnetic induction device with the circuit substrate to eventually result in contact with adjacent conductors.
Again in one preferred embodiment of the present invention, at least a portion of the winding member is an electric wire coated with a thermally fusible material, that is wound into a uniformly layered coil block, and is subsequently caked into a layered coil block by heating to fuse the thermally fusible material, said caked coil block being mounted on the bobbin. According to this embodiment, since the winding members prewound into the uniformly layered coil block is mounted on the bobbin, the winding member can readily and easily be mounted on the bobbin having a relatively small winding width as measured in a direction axially of the bobbin.
In an alternative embodiment of the present invention, the winding member includes primary and secondary windings and the primary winding has opposite lead lines that are connected with a primary circuit substrate included in the high frequency heating apparatus. The electromagnetic induction device may further include a secondary circuit substrate. The secondary winding is connected with the secondary circuit substrate. In this case, the bobbin is preferably formed integrally with a substrate mount for supporting the secondary circuit substrate.
According to this alternative embodiment, since the electromagnetic induction device has a flat configuration having a relatively small radial size, the integral provision of the secondary circuit substrate does not result in increase of the overall size thereof and does also allow the electromagnetic induction device in the form as separated from the primary circuit substrate to be installed at a relatively small space that may be chosen as desired from a vacant space available within the high frequency heating apparatus. Accordingly, if the electromagnetic induction device which would occupy a relatively large space on the circuit substrate is positioned at a suitable location separated from the circuit substrate, an apparatus equipped with such electromagnetic induction device, for example, the high frequency heating apparatus can advantageously be assembled compact in size. Moreover, since the primary circuit substrate electrically connected with the primary winding and the secondary circuit substrate connected with the secondary winding for generating a high voltage are separated from each other, a sufficient distance of insulation can be secured without incurring an increase in size of the space for installation.
Again in a further alternative embodiment of the present invention, the substrate mount is positioned laterally of the bobbin and radially outwardly of at least one of the primary and secondary windings. This design is particularly advantageous in that since the electromagnetic induction device according to the present invention has a relatively small radial size because of the absence of any core piece at a location radially outwardly of the bobbin, integration of the secondary circuit substrate with a lateral portion of the bobbin does not result in increase in size.
Also, the substrate mount may alternatively be formed in a collar that defines one axial end of the bobbin, and is positioned axially outwardly of the primary and secondary windings. This design allows the electromagnetic induction device to have a flat configuration and, therefore, even though the secondary circuit substrate is formed integrally with the color eventually forming one axial end of the bobbin, the electromagnetic induction device will not increase in size.
In a further preferred embodiment of the present invention, the bobbin may include a plurality of bobbin pieces defined by dividing the bobbin in a direction axially thereof and wherein each of the core pieces is embedded in the corresponding bobbin piece preferably by an insert-molding technique. Since in the electromagnetic induction device embodying the present invention, the core pieces are mounted on and integrated together with the respective bobbin pieces by the use of the insert-molding technique, this design is effective to eliminate the need to employ a manufacturing step of fixing the core pieces by a fixture such as a core clip after the latter have been assembled into the bobbin and, therefore, the number of the manufacturing steps can correspondingly be reduced along with reduction in number of component parts, resulting in reduction in manufacturing cost.
Preferably, at least a portion of outer surface of the core arm of each of the first and second core pieces on which outer surface no corresponding core leg is formed is exposed to an outside, so that heat evolved in the respective core piece embedded in the associated bobbin piece by the insert-molding technique can advantageously dissipated.
In a yet further preferred embodiment of the present invention, the bobbin may have at least one winding groove defined therein for receiving the winding member provided therein and may be made up of a plurality of bobbin pieces defined by dividing the bobbin in a direction axially thereof In such case, the plural bobbin pieces are to be connected together such that a groove width of the winding groove straddling the neighboring bobbin pieces is variable. According to this design, change of the groove width of the winding groove can effectively result in change in winding width of the winding member.
According to a still further preferred embodiment of the present invention, the bobbin may include at least first and second bobbin pieces each including a hollow cylindrical body having a throughhole defined therein. The bore is defined by the respective throughholes in the bobbin pieces when the respective hollow cylindrical bodies of the first and second bobbin pieces are coaxially aligned with each other. The bobbin pieces are assembled together to complete the bobbin with the hollow cylindrical body in the first bobbin piece inserted into the hollow cylindrical body in the second bobbin piece.
In this embodiment, one of an inner peripheral surface of the hollow cylindrical body in the first bobbin piece and an outer peripheral surface of the hollow cylindrical body in the second bobbin piece is formed with an engagement projection, and the other of the inner and outer peripheral surfaces of the hollow cylindrical bodies in the respective bobbin pieces is formed with an axially extending guide groove and a plurality of circumferentially extending engagement grooves communicated with the guide groove and spaced a distance from each other in a direction axially of the bobbin. Also, when the hollow cylindrical bodies of the first and second bobbin pieces are connected together one inserted into the other, the engagement projection is guided along the guide groove in the axial direction and is subsequently engaged in one of the engagement grooves upon relative displacement of the hollow cylindrical bodies in the circumferential direction. According to this structure, merely by selecting one of the engagement grooves to be engaged with the engagement projections, the width of the winding groove can be changed simply.